Sn4 Intermetallic Compounds Research Articles

First-Principles Study of Cu Addition on Mechanical Properties of Ni3Sn4-Based Intermetallic Compounds

Ni–Cu under-bump metallisation (UBM) can reduce stress and improve wetting ability in technology for electronic packaging technology advances with three-dimensional integrated circuit (3D IC) devices. The bond between the Sn-based solder and Ni–Cu UBM is affected by the formation of intermetallic compounds (IMCs), specifically Ni3Sn4 and (Ni,Cu)3Sn4. This paper investigates the mechanical properties of IMCs, which are critical in assessing the longevity of solder joints. First-principles calculations were carried out to investigate the phase stability, mechanical properties and electronic structures of Ni3Sn4, Ni2.5Cu0.5Sn4, Ni2.0Cu1.0Sn4, and Ni1.5Cu1.5Sn4 IMCs. The calculated formation enthalpies show that the doping of Cu atoms leads to a decrease in the stability of the phases and a reduction in the mechanical properties of the Ni3Sn4 crystal structure. As the concentration of Cu atoms in the Ni3Sn4 cells increases, the bulk modulus values of (Ni,Cu)3Sn4 formed with different compositions decrease from 107.78 GPa to 87.84 GPa, the shear modulus decreases from 56.64 GPa to 45.08 GPa, and the elastic modulus decreases from 144.59 GPa to 115.48 GPa, indicating that the doping of Cu atoms into the Ni3Sn4 cells may adversely affect their mechanical properties and increase the possibility of microcracking at the interface during actual service. The anisotropy of (Ni,Cu)3Sn4 is more significant than that of Ni3Sn4, with Ni2.0Cu1.0Sn4 showing the highest anisotropy. After evaluating the electronic structures, the metallic properties of Ni3Sn4 and the Ni2.5Cu0.5Sn4, Ni2.0Cu1.0Sn4, and Ni1.5Cu1.5Sn4 phases are revealed by electronic structure analysis. The total density of states (TDOS) for (Ni,Cu)3Sn4 structures is mainly influenced by Ni-d and Cu-d states. The addition of Cu atoms can increase the brittleness of Ni3Sn4. In addition, the region where d and p hybridisation occurs gradually increases with increasing Cu content. The electronic properties suggest that the binding energy between Ni and Sn atoms weakens with the addition of Cu atoms, resulting in a decrease in the elastic modulus. This research can serve as a valuable reference and theoretical guide for future applications of these materials.

Electromigration-induced remarkable intermetallic compound (IMC) formation in micro joints and its prevention

The electromigration reliability of Au/Pd(P)/Ni(P) surface finish in micro joints was investigated in this study. We found that the reaction of Pd(P) with molten Sn would yield a noticeable amount of (Pd,Ni)Sn4 intermetallic compound (IMC) scattering in the Sn matrix after soldering reaction. Interestingly, upon electron current stressing, (Pd,Ni)Sn4 was gradually driven to the anode interface, and a remarkable volume expansion was caused by the transition from a low Ni-containing (Pd0.22Ni0.78)Sn4 (17 μm on thickness) into a high Ni-containing (Pd0.04Ni0.96)Sn4 (56 μm on thickness) because of the substitution of Pd sublattice by a high fraction of Ni as a result of electromigration. This electromigration-induced unusual stoichiometric transition and massive IMC propagation has been rarely reported previously in the literature. The underlying mechanism associated with the electromigration phenomenon and mitigation strategies to the NiSn4 formation were proposed in this study.

Diffusion Barrier Properties of the Intermetallic Compound Layers Formed in the Pt Nanoparticles Alloyed Sn-58Bi Solder Joints Reacted with ENIG and ENEPIG Surface Finishes

Pt-nanoparticle (NP)-alloyed Sn-58Bi solders were reacted with electroless nickel-immersion gold (ENIG) and electroless nickel-electroless palladium-immersion gold (ENEPIG) surface finishes. We investigated formation of intermetallic compounds (IMCs) and their diffusion barrier properties at reaction interfaces as functions of Pt NP content in the composite solders and duration of solid-state aging at 100 °C. At Sn-58Bi-xPt/ENIG interfaces, typical Ni3Sn4/Ni3P(P-rich layer) microstructure was formed. With the large consumption of the Ni-P layer, the Ni-P and Cu layers were intermixed and Cu atoms spread over the composite solder after 500 h of aging. By contrast, a (Pd,Ni)Sn4/thin Ni3Sn4 microstructure was observed at the Sn-58Bi-xPt/ENEPIG interfaces. The (Pd,Ni)Sn4 IMC effectively suppressed the consumption of the Ni-P layer and Ni3Sn4 growth, functioning as a good diffusion barrier. Therefore, the Sn-58Bi-xPt/ENEPIG joint survived 500 h of aging without microstructural degradation. Based on the experimental results and analysis of this study, Sn-58Bi-0.05Pt/ENEPIG is suggested as the optimum combination for future low-temperature soldering systems.

Effective (Pd,Ni)Sn4 diffusion barrier to suppress brittle fracture at Sn-58Bi-xAg solder joint with Ni(P)/Pd(P)/Au metallization pad

In this study, we investigated the interfacial reactions and mechanical properties of Sn58Bi solders alloyed with Ag nanoparticles (NPs) after bonding with electroless nickel-immersion gold (ENIG) or electroless nickel-electroless palladium-immersion gold (ENEPIG) surface finish. On the ENIG surface finish, Ni3Sn4, Ni-Sn-P, and Ni3P layers were sequentially formed on the Ni(P) layer. On the ENEPIG surface finish, (Pd,Ni)Sn4 intermetallic compound was formed on thin Ni3Sn4/Ni3P on the Ni(P) layer, and it survived nine reflow cycles, unlike Sn-Ag-Cu solder joints in previous studies. This effectively stopped the formation of the Ni-Sn-P layer, which acted as an effective diffusion barrier to suppress the consumption of Ni in the Ni(P) layer. A mixed-mode fracture—partially through the bulk solder (ductile mode) and partially through Kirkendall voids in the Ni-Sn-P layer (brittle mode)—was observed with the Sn-58Bi-xAg/ENIG joint, whereas only ductile fracture was observed with the Sn-58Bi-xAg/ENEPIG joint. Alloying with 0.5 wt% Ag NPs strengthened the composite solder by refining the eutectic microstructure of the Sn58Bi solder and by inducing precipitation hardening with fine Ag3Sn particles. However, the addition of more Ag NPs led to mechanical degradation by re-coarsening of the eutectic microstructure and by providing an easy fracture path along the coarsened Ag3Sn/solder interface. The highest shear strength was obtained with Sn-58Bi-0.5Ag/ENEPIG, which was suggested to be the most reliable system for low-temperature soldering at 200 °C.

Effects of a phosphorous-containing Pd layer in a thin-ENEPIG surface finish on the interfacial reactions and mechanical strength of a Sn–58Bi solder joint

We analyzed the effects of phosphorous (P) in a palladium (Pd) layer on the interfacial reactions and mechanical properties of Sn–58Bi solder with thin electroless-nickel electroless-palladium immersion gold (ENEPIG) joints (pure Pd and Pd(P) joints) after a high-temperature storage test. Flake-type (Pd,Au)Sn4 intermetallic compounds (IMCs) mainly formed in the pure Pd and Pd(P) joints. After being aged at 85–115 °C for 300 h, top- and bottom-side (Cu,Ni)6Sn5 IMCs formed at the interface between the (Pd,Au)Sn4 IMC and partially destroyed P-rich Ni layer of the pure Pd joints. In contrast, a Ni3Sn4 IMC formed at the interface between the (Pd,Au)Sn4 IMC and P-rich Ni layer of the Pd(P) joints after most aging temperatures and durations. This is because the reaction rate of the Pd(P) joint was lower than that of the pure Pd joints. The unreacted Ni layer remained at the Pd(P) joints after aging. In a high-speed shear test, the shear strength of the pure Pd joints rapidly decreased after being aged at 115 °C for 100 h. In contrast, the shear strength of the Pd(P) joints slightly decreased after being aged at 115 °C for 1000 h. The shear strengths of the pure Pd and Pd(P) joints were quite different under same aging conditions because of the different diffusion rates and IMC compositions of the pure Pd and Pd(P) joints. Fracture surfaces of the pure Pd joints after being aged at 115 °C were observed on the Cu substrate, but those of the Pd(P) joints were on the Ni3Sn4 IMC surface. Therefore, the Sn–58Bi solder with a Pd(P) layer in thin-ENEPIG is expected to be more reliable than that with pure Pd joints under aging treatment.

Effect of Cu additions on mechanical properties of Ni3Sn4-based intermetallic compounds: First-principles calculations and nano-indentation measurements

The (Ni,Cu)3Sn4 phase as intermetallic compounds (IMCs) plays an important role in evaluating reliability of solder joints. The first-principles calculations were performed to investigate the phase stability, mechanical properties and electronic structures of Ni3Sn4 and Ni2.5Cu0.5Sn4 IMCs. The results shows that the doping of Cu atom not only reduced the phase stability but also weakened the mechanical properties of Ni3Sn4 crystal structure. Meanwhile, all studied Ni3Sn4-based structures were structural stability. Moreover, the anisotropy of Ni2.5Cu0.5Sn4 (2a site) was strongest while that of Ni2.5Cu0.5Sn4 (4i site) was the weakest. Based on analysis of electronic structures, it indicated that the ionic bonding of NiCu in Ni2.5Cu0.5Sn4 structure was weaker than covalent bonding of nearest-neighbor Ni in Ni3Sn4, which led to an energy reduction with the formation of banding states. By means of nano-indentation measurements, the young's modulus and hardness were measured to be 135.3 ± 8.4 GPa and 5.0 ± 0.63 GPa for Ni3Sn4 while 126.3 ± 7.6 GPa and 4.7 ± 0.72 GPa for (Ni,Cu)3Sn4, respectively, which were close to the value of calculated results.

The interfacial bilayer Cu6Sn5 formed in a Sn–Ag–Cu flip-chip solder joint incorporating Au/Pd metallization during solid-state aging

A solid-state reaction between the Sn–Ag–Cu solder and Ni metallization resulted in the formation of interfacial bilayer Cu6Sn5-based intermetallic compounds (IMCs) in a solder joint incorporating Au/Pd metallization. The layer near the Ni metallization was identified as (Cu,Ni,Au)6Sn5 containing 20.3 at.% of Ni and 1.7 at.% of Au, and the layer near the solder matrix was identified as (Cu,Au,Ni,Pd)6Sn5 containing 5.7 at.% of Au, 1.2 at.% of Ni, and 1.0 at.% of Pd. The electron diffraction analysis with high resolution transmission electron microscopy further characterized the interfacial bilayer IMCs as having the same hexagonal crystal structure with different crystal orientations. The kinetics study revealed that the (Cu,Ni,Au)6Sn5 in the interfacial bilayer reaction products was formed during the initial reflow process, while the (Cu,Au,Ni,Pd)6Sn5 layer was formed during subsequent solid-state aging. Predominant growth of the (Cu,Au,Ni,Pd)6Sn5 layer was observed during solid-state aging. In contrast, the growth of the initially formed (Cu,Ni,Au)6Sn5 layer was suppressed. The growth of the (Cu,Au,Ni,Pd)6Sn5 layer was governed by the dissolution of the (Au,Pd)Sn4 IMC that occurred during solid-state aging. The complete dissolution of (Au,Pd)Sn4 resulted in a reactant-limited chemical reaction from the conversion of (Au,Pd)Sn4 to (Cu,Au,Ni,Pd)6Sn5. The development of the interfacial bilayer IMCs in the solder joint incorporating Au/Pd metallization suppressed excessive IMC growth and unfavorable phase transformation during long-term solid-state aging. The detailed mechanism of the formation of the interfacial bilayer Cu6Sn5-based IMCs was investigated in this paper.

Solid-state growth kinetics of intermetallic compounds in Cu pillar solder flip chip with ENEPIG surface finish under isothermal aging

Electroless Ni/electroless Pd/immersion Au (ENEPIG) constitutes a promising metallization to replace the conventional ENIG or Ni/Au for 3D IC integrated module and optoelectronic packaging. Different ENEPIG plating thicknesses with copper pillar solder joints are subjected to thermal aging at 150 °C to investigate the intermetallic compounds (IMCs) formation and growth. Due to low temperature solid-state bonding, both Pd and Au layers do not completely diffused into Sn matrix and participated in the interfacial reactions to form (Pd,Au)Sn4 IMCs phase. (Pd,Au)Sn4 IMCs based on PdSn4 phase with dissolved Au, exhibits high growth rate and substantial consumption of Sn from solder. Upon increasing the aging time, it is found that (Cu,Ni)6Sn5 IMCs and (Pd,Cu,Au)Sn4 phase form at interface and follows a diffusion control mechanism, which weakened the solder joint and caused brittle fracture in a die peel test. (Cu,Ni)6Sn5 IMCs growth rate increases with decreases Pd thickness. Therefore, the types of IMCs formation, growth rate, and reliability of Cu pillar joint strongly depend on bonding temperature, Au and Pd thicknesses, and solder volume. Based on this study, the authors recommend suitable ranges of Au and Pd layer thicknesses for reliable Cu pillar solder joints.

Interfacial Reaction and Mechanical Characterization of Sn–Ag–Cu/Au/Pd(P)/Cu Solder Joints: Thick Pd(P) Deposition

The solderability of an Au/Pd(P)/Cu trilayer structure (thickness 0.15 μm/0.6–1.0 μm/20 μm) was investigated by use of a scanning electron microscope equipped with an electron backscatter diffraction system, electron probe microanalysis, and high-speed ball shear (HSBS) testing. Thick Pd(P) deposition, particularly the structure with a Pd(P) thickness of 1.0 μm, resulted in a substantial amount of (Pd,Cu)Sn4 intermetallic compound in the solder matrix after reflow. This phase gradually resettled at the interface and formed a dense layer of (Pd,Cu)Sn4 over (Cu,Pd)6Sn5 during solid-state reaction. A Cu–Pd–Sn isotherm was examined to explain the formation and resettlement of (Pd,Cu)Sn4. The (Pd,Cu)Sn4/(Cu,Pd)6Sn5 dual layer at the interface was a crack initiation site in HSBS testing, and led to substantial degradation of the mechanical strength of solder joints, indicating that thick Pd(P) deposition should be avoided to prevent the Pd-induced embrittlement.

Effect of Thermal Aging on the Microstructure Evolution and Solder Joint Reliability in Hard Disk Drive Under Mechanical Shock

The effect of thermal aging on the microstructure evolution and solder joint reliability in hard disk drive (HDD) under mechanical shock was investigated. Significant coarsening of Ag3 Sn particles was found in SnAgCu solder, and AuSn4 intermetallic compound (IMC) changed from needle-type to layer-type during aging. For as-soldered SnAgCu solder joints after mechanical shock, the cracks were initiated in AuSn4 at the corner of the solder joints, and mainly propagated along the thin Ni3 Sn4 IMC layer. After aging at 150 degC for 21 days, the cracks were mainly propagated along the solder, Ni3 Sn4, Au-Sn-Ni-Cu, and Au-Cu-Sn. The significant coarsening of microstructure was found in SnPb solder joints, and only microcracks were found on the surfaces of as-soldered and aged solder joints after mechanical shock.

Ag–Copper Structure for Bonding Large Semiconductor Chips

Ag-copper dual-layer substrate design is presented. The Ag cladding on the copper substrate is a buffer to deal with the large mismatch in coefficient of thermal expansion (CTE) between semiconductors such as Si (3 ppm/degC) and Cu (17 ppm/deg C). Ag is chosen because of its low yield strength, only one-tenth of that of Cu and one-third of the popular Sn3.5Ag solder. Other advantages are high electrical conductivity and high thermal conductivity. To bond Si chips to the Ag layer on copper substrates, Sn-rich solder is used. A fluxless bonding process is designed and developed. The bonding media are Ni/Sn/Au multilayer solder structure plated over Ag. In this design, Ni is a diffusion barrier between Sn and Ag. The thin (100 nm) outer Au layer prevents inner Sn from oxidation. The Si chip is deposited with Cr/Au under bump metallurgy (UBM). The bonding process is performed in 50-mtorr vacuum atmosphere without any flux. Comparing to bonding in air, the oxygen content is reduced by a factor of 15 200. The resulting joints consist of three distinct layers, i.e., Sn-rich layer, Ni3 Sn4 intermetallic compound, and Ni. Scanning acoustic microscopy (SAM) is used to verify the quality of the joint. Microstructure and composition of the joints are studied using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX). This technique presents an initial success in overcoming the very large mismatch in thermal expansion between silicon and copper. It can be applied to mounting numerous high-power silicon devices to Cu substrate for various applications such as hybrid automotive and high-voltage power networks.

Effects of Solder Volume on Formation and Redeposition of Au-Containing Intermetallics in Ni/Au-SnAgCu-Ni(P) Solder Joints

SnAgCu is one of the most promising candidates for lead-free solders to replace conventional eutectic SnPb solders. The effects of solder volume on interfacial reactions and microstructure evolution in Ni/Au-SnAgCu-Ni(P) solder joints have been investigated under soldering and thermal aging conditions. The results show that solder volume has a strong effect on the formation of Au-containing intermetallic compounds (IMCs) and their redeposition at the interfaces. The size and volume fraction of Au-containing IMCs dispersed in the solder bulk increased with decreasing solder joint dimensions. For the small solder joint with 300-μm solder ball, the (Au,Ni)Sn4 IMCs redeposited to the interfaces after thermal aging at 150°C for 9 days, but this was not observed for the other two solder joints with large solder volume. These results also indicate that the redeposition of (Au,Ni)Sn4 is closely associated with the migration of Cu toward the interfaces.

Mechanical Properties of Intermetallic Compounds in the Au–Sn System

The mechanical properties of intermetallic compounds in the Au–Sn system were investigated by nanoindentation. Measurements of hardness and elastic modulus were obtained for all of the confirmed room-temperature intermetallics in this system as well as the β phase (8 at.% Sn) and AuSn4. Overall, it was found that the Au–Sn compounds have lower hardness and stiffness than common Cu–Sn compounds found in solder joints. This finding is in contrast to common knowledge of “Au embrittlement” due to the formation of either AuSn4 or (Au,Ni)Sn4 intermetallic compounds. This difference in understanding of mechanical properties of these phases and the resulting joint strength is discussed in terms of reliability and possible failure mechanisms related to interface strength or microstructural effects. Indentation creep measurements performed on Au5Sn, Au–Sn eutectic (29 at.% Sn) and AuSn indicate that these alloys are significantly more creep resistant than common soft solders, in keeping with typical observations of actual joint performance.